“Occasionally this happens, but that’s because we designed it this way,” Bill Utter told the Sun Times. “Whatever happened last night is a result of the protective coating doing what it’s designed to.”

Alejandro Garibay, along with his brother and two California cousins, shot video and photos of the cracking coating. The sound of cracking could be faintly heard on the video.

Garibay told NBC Chicago that he and his family were joking with workers at Willis Tower about whether the boxes would hold people up. After seeing the cracking and getting out of the glass boxes, he notified those workers, who had said the boxes were totally safe.

"I walked them over so they could see and they were totally shocked and asked us to step away and then proceeded to start calling staff and techs and I don't know who else. When we pulled our phones to start recording and take pictures they asked us to leave right away," he told NBC Chicago.

All four of the glass ledges at Willis Tower were closed for inspection Thursday morning, the Sun Times said.

Utter told the Times the coating has cracked in the past, sometimes caused when a sharp object hits the glass.